1. Field of the Invention
The invention relates generally to electrical apparatus, and, more particularly, to adjustable time delay relays employing solid state electronic components to establish the delay period.
2. Description of the Prior Art
Time delay relays are known which use a variety of components including solid-state, electromechanical, thermal, and pneumatic devices to establish a delay period. Only a limited number of types are available, however, which can operate in the absence of power. An application requiring such a time delay relay is an automatic transfer switch for connecting an electrical load to an emergency power source upon failure of the normal power source. Typically, a diesel-powered generator is provided on a standby basis to be started upon failure of the utility supply. The diesel engine is started after a suitable time delay to insure that the failure of the utility supply is not merely a transient condition. Thus, there often is no power available during the time delay period.
One time delay relay commonly used in this application employs a pneumatic device having a spring-loaded dashpot to operate a pair of relay contacts. Care must be taken, however, to ensure that the air orifice which determines the delay interval is free of dust and other contaminants. In addition, a pneumatic time delay relay has a relatively high cost.
A device using electrical components to produce a time delay typically employs a DC relay having a coil shunted by a large storage capacitor energized from the power line through a diode. Such a relay will energize at approximately 75% of rated voltage. The time constant of the capacitor and relay coil resistance determines the length of time required following failure of the power supply for the relay contacts will drop out. Since a DC relay typically drops out at 15 to 20% of normal operating voltage, the time delay is approximately 2 time constants of the storage capacitor and relay coil resistance. With a 120 microfarad capacitor and a 10,000 ohm relay coil, the delay will be approximately 21/2 seconds. Longer time delays may be obtained by increasing the value of the capacitor.
Such a timer is relatively low in cost. However, the delay period is dependent on the line voltage at the time of power supply failure. In an automatic transfer switch, line voltage can drop to as low as 70% of normal voltage before actual failure of the line, thus producing a substantial variation in the delay period. In addition, the large capacitors required to obtain the desired delay periods have a wide tolerance. The combination of these two factors can produce a delay period variation of as much as 250% of the design value. It is also quite difficult to provide an adjustable delay period using such a device.
It would therefore be desirable to provide a time delay relay which combines the low cost and lower maintenance requirements of the electrical delay timer with the accuracy of a pneumatic timer.